The present invention relates to improvements in a hydraulic motor. More specifically, the present invention relates to a hydraulic motor coupled to a pump employing a reciprocating piston. One use for such motors is to supply slurries of paint or other coating compositions to the several spray heads of an airless paint sprayer system.
Hydraulic motors used with reciprocating pistons for airless paint sprayers have been employed in the past. One known manufacturer of such a hydraulic motor is The Speeflo Division of Titan Tool, Inc. of Roslyn, N.Y. Other known manufacturers of hydraulic motors for airless paint sprayer systems include Graco, Inc. of Minneapolis, Minne., Durotech Co. of Moorpark, Calif. and Airlessco of Orange, Calif. A known Speeflo pump design of Titan employs a differential area piston with a spool valve to drive the pump up and down. Hydraulic pressure is always directed to the bottom side of the piston, and the valve alternately applies and removes pressure from the larger top side of the piston. A pair of ball detent latching mechanisms serves to maintain the spool valve in one of two possible end positions.
One of the problems with the known Speeflo pump design is that the spool rides in a spool sleeve which must match the spool diameter within about 0.0003 inches. When any wear occurs at the interface between the spool and the sleeve, leakage from the high pressure supply side to the no pressure exhaust side increases rapidly. This leakage causes frictional heating of the hydraulic fluid and lowers the pump's performance, i.e. its maximum pressure and flow rate. In addition, the ball riding over the detent ridge on the spool causes wear on both parts. The springs loading the ball eventually also fail, generating debris which can damage the spool valve. Typically, two balls are used in opposition to each other on the spool. While this keeps the spool centered between the two--and is a better design than employing a single spring loaded ball valve--the two balls still try to force the spool to move off axis, in a direction perpendicular to the ball loading. Such angulation causes wear on both the spool and the sleeve.
In addition, whether the spool needs replacing due to wear on its detent ridge from the ball, or due to scoring from debris, both the spool and the sleeve must be replaced as a set. In order to achieve the tight clearance that is necessary between the spool and the sleeve, they need to be made in sets.
Ideally, the shift distance for the pump should be as small as possible in order to maximize efficiency, limited only by the flow area requirements for fluid passages and to allow for a seal between the two sides. However, the use of a ball detent increases the minimum shift distance due to the space required for the ball to be in both the up and the down positions and the size of the detent ridge. Larger balls in the ball detent mechanism and larger detent ridges are desirable to lower the stresses on and to improve the life of the latching mechanism. But, they also lengthen the shift distance and reduce the efficiency of the pump. In addition, the movement of the ball during the shift from one end position to the opposing end position in the pump, as the trip rod spring loads up, contributes to a longer shift time, during which the spool is not in either a fully up or a fully down position.
Another known hydraulic motor for an airless paint sprayer system is disclosed in U.S. Pat. No. 4,785,997 dated Nov. 22, 1988 and assigned to Durotech Co. of Moorpark, Calif. This hydraulic motor includes a releasable stop means having balls and grooves wherein the construction of the same will assure a rotation of the balls to prevent uneven wear of the balls and the grooves. However, since this known arrangement also employs balls and grooves, it has the same disadvantages as mentioned above with regard to the Speeflo design.
Accordingly, it is desirable to develop a new and improved hydraulic motor which would overcome the foregoing difficulties and others while providing better and more advantageous overall results.